1. Field of the Invention
This invention relates to piezoelectric sensors such as AE (acoustic emission) sensors, ultrasonic sensors, vibration sensors, etc. using piezoelectric materials.
2. Description of Related Art
Piezoelectric sensors for converting slight vibrations and stress of objects under measurement into electric signals with the piezoelectric effect of their materials find applications in various fields. In an example of an application, the piezoelectric sensor is mounted in a mechanically operable apparatus for detecting abnormal vibrations that may be caused when an abnormal operation takes place in the machine.
The inventor has proposed a piezoelectric sensor which is used in this way. FIGS. 12 and 13 show this proposed sensor, but the sensor has not been known or sold in Japan and foreign country. Specifically, FIG. 12 is a schematic sectional view showing the overall structure of the piezoelectric sensor, and FIG. 13 shows a piezoelectric element provided in the piezoelectric sensor in a top view, a bottom view and a sectional view taken along line I--I in the top view.
The piezoelectric sensor 11 comprises a piezoelectric element 13, a support (i.e., a vessel) 17, to which the element 13 is secured by securing means 15, e.g., an adhesive, inner leads 19a connected to a first and a second electrode 13b and 13c of the element 13, outer leads 19b connected to the inner leads 19a, and a lid 21 of the vessel 17.
In this example, the piezoelectric element 13 includes a piezoelectric body 13a having a disk-like shape, the first and second electrodes 13b and 13c being provided on the opposite surfaces of the piezoelectric body 13a. In the instant piezoelectric element 13, a portion 13d of the second electrode 13c is on the surface of the body 13a, and the first electrode 13b of the piezoelectric body 13a is also formed on that same surface. The piezoelectric body 13a is often made of piezoelectric ceramics, for instance of lead zirconate titanate. The piezoelectric body 13a has such dimensions that it readily resonates at the frequency of abnormal vibration desired to be detected. The first and second electrodes 13b and 13c are often made of sintering of silver paste.
The vessel 17 is often made of iron because it requires only an inexpensive material expenditure.
The inner leads 19a are often made from a wire having a very small diameter in order to alleviate as much as possible the transfer of its stress to the piezoelectric element 13. The outer leads 19b are often made from a sielded wire thicker than the inner leads 19a, which is advantageous from the standpoints of the durability and measures against noise. One end of the outer leads 19b is secured to the lid 21 and connected to the inner leads 19a in the neighborhood of the lid 21.
However, as a result of extensive research and investigation conducted by the inventor in connection with this application, it has been found that the already proposed piezoelectric sensor as described above has the following problems.
(1) An increase of the temperature range of the working environment, in which the piezoelectric sensor is used, will increase the temperature dependency of the sensitivity of the piezoelectric sensor (comparative examples described later being referred to for details). Therefore, it becomes impossible to obtain measurements as described.
(2) Since the wiring has a structure requiring separate inner and outer wirings, its production is cumbersome. In addition, it is difficult to ensure reliability of the connection between these two wirings.
(3) The inner leads have to be made from a very small diameter wire, and therefore their ability to withstand vibration (durability) is inferior.
(4) To alleviate the influence of the temperature, it is thought to use the piezoelectric sensor in a state accommodated in a suitable protector. However, when such a protector is used, the piezoelectric sensor is correspondingly increased in shape. In addition, the sensitivity is reduced.
A piezoelectric sensor used in a stringent environment, for instance one for detecting abnormal operation of an apparatus installed in an automotive engine compartment, is required to permit measurements in a broad temperature range, for instance from -40.degree. to +150.degree. C., as well as being durable and small in size. Therefore, the solution of the above problems has been desired.